Bone anchors may be used in orthopedic surgery to fix bone during healing or during a fusion process. In spinal surgery, bone anchors may be used with spinal fixation elements, such as spinal rods, to stabilize multiple vertebrae either rigidly, in which no relative motion between the vertebrae is desired, or dynamically, in which limited, controlled motion between the vertebrae is desired. Fixation elements can help to support the spine in a desired alignment, for example by defining a shape towards which a deformed spine is to be corrected. Attaching the vertebrae to the fixation element causes vertebrae which are out of position to be drawn towards the fixation element, so that they can then be retained in a correct alignment against forces imposed by soft tissue tending to revert the configuration of the spine towards the deformed shape. Correction of the spinal deformation can involve application to the vertebrae of translational forces, torsional forces, or combinations thereof to cause vertebrae to translate and/or rotate.
Traditional bone anchor assemblies include a bone anchor having a rod-receiving member formed thereon or coupled thereto for seating a spinal fixation rod. A compression member disposed distally of the spinal fixation rod and proximally of the bone anchor can be provided for locking the bone anchor at a fixed angular orientation relative to the rod-receiving member. A closure mechanism disposed proximally of the spinal fixation rod and can be provided to lock both the bone anchor and the spinal fixation element within the rod-receiving member. While the use of a compression member in combination with a closure mechanism can be an effective means to secure the assembly, traditional compression members and closure mechanisms can be prone to slip relative to the bone anchor, the rod-receiving member, and/or relative to one another. Slippage of either of these components can cause the bone anchor and/or the spinal fixation rod to move from a desired orientation within the rod-receiving member and can therefore compromise the effectiveness of the bone anchor assembly for correcting spinal deformities.
Accordingly, there remains a need for improved methods and devices for bone anchor fixation.